Interpretive Summary: Resistance to modern anthelmintics by ruminant nematode parasites is an increasing problem throughout the world. To date the problem has largely been reported in parasites of small ruminants, but there are increasing reports of such resistance in nematodes recovered from cattle. Until now there have been no published reports of drug resistant parasites from cattle in North America. In 2002 a producer in the upper Midwest who backgrounds young cattle acquired from the southeastern U.S. experienced lower than expected weight gain as well as what appeared to be illness caused by parasites the fall. Fecal sample results supported the idea that decreased productivity was the result of GI nematode parasitism. The operation used intensive grazing management and practiced strategically timed deworming for more than 17 years. To assess whether the apparent decreased drug efficacy was the result of drug resistance in the nematode population, in 2003, approximately 150 head, previously strategic timed dewormed, of 9-11 month old cattle from one pasture were selected for study. The calves were treated with ivermectin injectable(I), moxidectin pour-on(M), doramectin injectable(D), eprinomectin pour-on(E), albendazole oral(A) or left untreated (U). Drugs were administered as directed on the label. Seven days later, 3 calves from each treatment group were slaughtered for worm recovery. Fecal samples were taken from the remaining animals at 14 days post treatment showed that the percent reduction of mean fecal EPG value for each group was: U-46%, I-52%, M-72%, D-61%, E-8%, and A-68%, all values well below those demonstrated when the drugs were first aopproved for use in cattle. Worms recovered from the killed animals showed significant numbers of Haemonchus placei and H. contortus in the stomach, and significant numbers of Cooperia punctata, and smaller numbers of C. oncophora, and C. spatulata in the intestine. Further the results showed that the pastures harbored substantial numbers of H. contortus resistant to both avermectins and benzimadoles, and substantial numbers of H. placei and Cooperia sp. resistant to all the commonly used avermectin anthelmintics. This is the first report of anthelmintic resistance in American cattle parasites and indicates that producers may have to substantially change current management practices.

Technical Abstract:
Resistance to modern anthelmintics by ruminant nematode parasites is an increasing problem throughout the world. Until now there have been no published reports of drug resistant parasites from cattle in North America. In 2002 a producer in the upper Midwest who backgrounds young cattle acquired from the southeastern U.S. experienced lower than expected weight gain as well as what appeared to be parasitic gastroenteritis in his cattle during the fall. Fecal sample results supported the idea that decreased productivity was the result of GI nematode parasitism. The operation used intensive grazing management and practiced strategically timed deworming for more than 17 years. In 2003, all animals were dewormed the first week of May with Ivomec Plus, then with Dectomax Injectable on 4 June and 17 July. On 31 July, ten randomly taken fecal samples showed the EPG values from 0 to 55. To assess whether the apparent decreased drug efficacy was the result of drug resistance in the nematode population, on 18 Aug approximately 150 head, previously strategic timed dewormed, of 9-11 month old cattle from one pasture were selected for study. The calves were ranked and blocked based upon their EPG value using a modified Wisconsin flotation assay and randomly assigned to 1 of 6 treatment groups. Treatment groups were as follows: untreated (U), ivermectin injectable(I), moxidectin pour-on(M), doramectin injectable(D), eprinomectin pour-on(E), albendazole oral(A). All cattle were weighed prior to treatment and the drug was administered at the labeled dosage according to label directions. Seven days later, 3 calves from each treatment group were slaughtered for worm recovery. Fecal samples were taken from the remaining animals at 14 days post treatment showed that the percent reduction of mean fecal EPG value for each group was: U-46%, I-52%, M-72%, D-61%, E-8%, and A-68%. Calves for slaughter were chosen by their EPG rank, i.e. 1 block of 6 calves encompassing all treatments near the median EPG value, 1 block near the high and 1 block near the low. Worms were recovered from washes of the abomasum and small intestine and from an 4 hour incubation in warm saline of the abomasum. Separate 20% aliquots from each sample were processed at 2 different locations and the mean of these counts was used as an indication of parasite numbers. The abomasum of all groups harbored significant numbers of Haemonchus placei and H. contortus. The results for each treatment group presented as total worms/ % H. placei/ % H. contortus were: U = 1582/29/71 ; I = 2616/28/72; M =1950/33/67 ; D = 2034/27/73; E = 2019/45/55; and A = 2291/0/100 . The small intestines from all avermectin treated groups contained significant numbers of Cooperia punctata, and smaller numbers of C. oncophora, and C. spatulata . The results for each treatment group presented as total worms/ % C. punctata/ C. oncophora/ % C. spatulata were U = 242/89/0/11 ; I = 11875/90/8/2; M = 5308/96/0/4; D = 6482/86/10/3; E = 3712/85/2/13; and A = 17/100/0/0. These results imply that the pastures studied contain substantial numbers of H. contortus resistant to both avermectins and benzimadoles, and H. placei and Cooperia sp. resistant to all the commonly used avermectin anthelmintics. This is the first report of anthelmintic resistance in American cattle parasites.